Microelectronic devices mounted on a circuit board often require long-term, independent battery power. For instance, SRAM memory requires a replaceable battery to backup information and can benefit significantly from a long-term, maintenance-free power source. Furthermore, the goal of a long-term, radioisotope-based, energy harvesting power source for microelectronic components has been sought for decades.
U.S. Pat. No. 5,642,014 discloses a self-powered semiconductor device in which a radioactive power source and an integrated circuit (IC) are formed on a substrate. The method of forming both components on the same substrate requires the integrated circuit manufacturer to substantially change its die configuration, thereby increasing the cost to the end-user or perhaps precluding use of the radioactive power solution altogether. For instance, an integrated radioactive power source on the IC can be problematic if radiation affects the device's performance by inducing bit flips in the bit stream or in memory. Furthermore, the radioactive power source may not provide sufficient power to the integrated circuit or SRAM due to the constraint of using only a minute area of the IC for the power source, while also being limited by the IC's substrate configuration.
U.S. Pat. No. 6,998,692 discloses a betavoltaic power source that is integrated into an IC package but separated from the IC's substrate and separately mounted into the IC package. This formulation allows for some reduction of the constraints present in U.S. Pat. No. 5,642,014; however, this design also suffers from integration problems with the IC manufacturing process, thereby increasing the cost to the end-user. This cost increase may preclude the use of a radioactive power source, if the IC manufacturer is not willing to integrate a radioactive power source into the IC package. The integrated circuit is also prone to radiation effects that are dependent upon the nature of the selected radioisotope species, shielding requirements, and proximity of the IC and power source within the IC package.
In the case of field programmable gate arrays (FPGAs), the highest level of encryption security is dependent upon battery-backed SRAM to maintain encryption keys. Unfortunately, the dependability of conventional chemical-based batteries, e.g. lithium, nickel-cadmium, alkaline cells, has reduced the reliability of encryption security in FPGAs. This reliability issue has necessitated the frequent replacement of batteries on circuit boards to maintain FPGA encryption security. This replacement feature has caused an inefficient use of circuit board real estate due to the introduction of special component holders for chemically-based coin-cell batteries and other form-factor batteries. In addition, requirements for chemical battery replacement necessitate easy operator-access to the battery on the circuit board. This has created a fundamental weakness in encryption security due to the presence of exposed (and easily accessible) battery leads, thereby making such configurations more susceptible to reverse engineering attacks.
Those skilled in the art of reverse engineering have recently found that a reduced level of security exists when the FPGA's encryption key power lead connections are easily accessible; such accessibility leads to an increased susceptibility to tamper/reverse engineering attempts.
The disadvantages associated with the use of chemical-based batteries can be surmounted through use of radioisotope power sources, which are gradually being introduced into the microelectronics market. However, the radioisotope power sources also introduce disadvantages that must be overcome to secure their widespread adoption.